Strongly coupled nematicity and magnetism in FeSe

Figure: Pressure-temperature phase diagram of FeSe.
Figure: Pressure-temperature phase diagram of FeSe.

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. Using HPCAT capabilities of high-pressure x-ray diffraction and time-domain Mössbauer spectroscopy, the nematicity and magnetism in FeSe under applied pressure are found to be strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≳1.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1−xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors. [K. Kothapalli et al, Nature Comm. 7, 12728 (2016)]

 

Figure: Pressure-temperature phase diagram of FeSe. Phase regions are labelled with the orthorhombic (OR), magnetic (M) and superconducting (SC) ordered states. The transition temperatures obtained from the present single-crystal HE-XRD measurements (red symbols) and fits of the NFS data (blue symbols, with error bars estimated from the width of the temperature steps in the measurement). Thick lines in the figure represent first-order phase transitions and thin lines correspond to second-order phase transitions. The insets to the figure show representative two-dimensional diffraction data in the respective pressure–temperature region demonstrating the splitting of the tetragonal (660) Bragg peak in the orthorhombic phase.